Process for making uniform aluminum oxide coating

Abstract

A process of making a dense, cohesive and uniform aluminum oxide coating on a metallic substrate includes electrodepositing polynuclear aluminum oxide hydroxide clusters from a polynuclear aluminum oxide hydroxide cluster solution on a metallic substrate to form a precursor coating, and post-treating the precursor coating to form a final aluminum oxide coating on the metallic substrate.

The invention claimed is: 
     
       1. A method comprising:
 electrodepositing polynuclear aluminum oxide hydroxide clusters from a polynuclear aluminum oxide hydroxide cluster solution on a metallic substrate to form a precursor coating; and 
 post-treating the precursor coating to form an aluminum oxide coating on the metallic substrate. 
 
     
     
       2. The method of  claim 1 , wherein electrodepositing polynuclear aluminum oxide hydroxide clusters comprises:
 filling an electrochemical cell with the polynuclear aluminum oxide hydroxide solution comprising polynuclear aluminum oxide hydroxide clusters and counter-anions, and wherein the electrochemical cell comprises the metallic substrate and an anode; 
 applying a voltage across the electrochemical cell such that the metallic substrate is negative and the anode is positive; 
 electrodepositing a first layer of polynuclear aluminum oxide hydroxide clusters from the polynuclear aluminum oxide hydroxide solution onto the metallic substrate; and 
 consolidating the polynuclear aluminum oxide hydroxide clusters on the metallic substrate. 
 
     
     
       3. The method of  claim 1 , further comprising electrodepositing a second layer of polynuclear aluminum oxide hydroxide clusters on to the metallic substrate. 
     
     
       4. The method of  claim 1 , wherein the metallic substrate is comprised of a material selected from the group consisting of aluminum, titanium, zirconium, hafnium, niobium, tantalum, tungsten, magnesium, zinc, iron, and alloys thereof. 
     
     
       5. The method of  claim 1 , further comprising synthesizing the polynuclear aluminum oxide hydroxide cluster solution. 
     
     
       6. The method of  claim 5 , wherein synthesizing the polynuclear aluminum oxide hydroxide cluster solution comprises electrolysis of an aluminum anode in an acidic solution. 
     
     
       7. The method of  claim 5 , wherein synthesizing the polynuclear aluminum oxide hydroxide cluster solution comprises:
 dissolving an aluminum salt into an aqueous solution; 
 forming polynuclear aluminum oxide hydroxide clusters by controlled neutralization; and 
 coagulating the polynuclear aluminum oxide hydroxide clusters. 
 
     
     
       8. The method of  claim 7 , wherein the aluminum salt is selected from the group consisting of aluminum nitrate, aluminum chloride, aluminum triacetate, and aluminum tri-sec-butoxide. 
     
     
       9. The method of  claim 7 , further comprising re-suspending the polynuclear aluminum oxide hydroxide clusters in aqueous solution or dispersing the polynuclear aluminum oxide hydroxide clusters in an organic solvent. 
     
     
       10. The method of  claim 7 , further comprising creating a sol with the polynuclear aluminum oxide hydroxide clusters. 
     
     
       11. The method of  claim 7 , wherein forming polynuclear aluminum oxide hydroxide clusters by controlled neutralization is done by titration with a base. 
     
     
       12. The method of  claim 1 , wherein electrodepositing polynuclear aluminum oxide hydroxide clusters comprises cathodic electrodeposition conducted at a voltage between 100 and 1000 volts such that micro-arc conditions form the precursor coating. 
     
     
       13. The method of  claim 5 , wherein the polynuclear aluminum oxide hydroxide solution further comprises a surfactant, binder, pH buffer, co-coagulant, corrosion inhibitor, chelating agent, rheology modifier, or monomeric metal ion. 
     
     
       14. The method of  claim 13 , wherein the polynuclear aluminum oxide hydroxide solution further comprises a co-coagulant selected from the group consisting of metal oxide salts containing silicon, molybdenum, tungsten, vanadium, iron, chromium, zinc, or copper, where the metal to aluminum ratio ranges from 1.5 to 15.0.